Methods and apparatus for detecting ventricular depolarizations during atrial pacing

ABSTRACT

AV synchronous, dual chamber pacing systems are disclosed having improved sensing of ectopic ventricular depolarizations or PVCs coincidentally occurring at or shortly following delivery of an A-PACE pulse. A first ventricular sense amplifier that is blanked during and following delivery of an A-PACE pulse is coupled to active and indifferent ventricular pace/sense electrodes defining a ventricular sense vector for sensing natural ventricular depolarizations and declaring a V-EVENT. A far field PVC sense amplifier coupled to a far field PVC sense electrode pair defining a PVC sense vector detects such PVCs while the ventricular sense amplifier is blanked. A PVC declared during the ventricular blanking period by the far field PVC sense amplifier is employed to deliver a VSP pulse upon time-out of a VSP delay, if the VSP function is provided and programmed ON, and/or to halt time-out of an AV delay.

RELATED APPLICATIONS

[0001] This disclosure is related to the following co-pending U.S.patent application Ser. No. (P-10266) filed Sep. 30, 2002, under Ser.No. 10/260,984, entitled “METHOD AND APPARATUS FOR PERFORMINGSTIMULATION THRESHOLD SEARCHES” by C. M. Manrodt et al., which is notadmitted as prior art with respect to the present disclosure by itsmention in this section.

FIELD OF THE INVENTION

[0002] This invention relates to implantable AV synchronous, dualchamber pacing systems, and particularly to improved sensing of ectopicventricular depolarizations coincidentally occurring at or shortlyfollowing delivery of an atrial pacing pulse.

BACKGROUND OF THE INVENTION

[0003] Atrial synchronized, dual chamber, pacing modes, particularly,the multi-programmable, VDD, VDDR, DDD and DDDR pacing modes, have beenwidely adopted in implantable dual chamber pacemakers for providingatrial and ventricular or AV synchronized pacing on demand. Such dualchamber pacing modes have also been incorporated into implantablecardioverter/defibrillators (ICDs) and into right and left heart pacingsystems providing synchronized right and left heart pacing for enhancingleft ventricular cardiac output as described in commonly assigned U.S.Pat. No. 5,902,324.

[0004] Such pacing systems are embodied in an implantable pulsegenerator (IPG) adapted to be subcutaneously implanted and at leastatrial and ventricular pacing or cardioversion/defibrillation leads thatare coupled to the IPG. The atrial and ventricular leads eachincorporate one or more lead conductor that extends through the leadbody to an exposed pace/sense electrode or cardioversion/defibrillationelectrode disposed in operative relation to a heart chamber. Typically,a negative-going or cathodal voltage pacing pulse is applied through apacing path comprising a small surface area, active pace/sense electrode(also characterized as a cathode electrode) and a relatively largersurface area, return or indifferent pace/sense electrode (alsocharacterized as an anode electrode) to pace a heart chamber.

[0005] Such leads are typically characterized as unipolar leads if theycomprise only a single active pace/sense electrode and/or acardioversion/defibrillation electrode. In the pacing context, aunipolar lead is coupled with a unipolar IPG, wherein the electricallyconductive IPG housing or “can” comprises a return or indifferentpace/sense electrode or anode electrode. Unipolar pacing and sensingtakes place between the lead-borne active pace/sense electrode and thehousing indifferent pace/sense electrode. A bipolar lead comprises atleast two lead conductors coupled to a bipolar IPG and extending to anactive pace/sense electrode, typically located at the distal end of thelead body, and an indifferent pace/sense electrode, typically located onthe lead body proximal to the distal active pace/sense electrode.Bipolar pacing and sensing takes place between the lead-borne activepace/sense electrode and indifferent pace/sense electrode. In thebipolar configuration, the indifferent pace/sense electrode is usually aring-like structure, referred to as the “ring” electrode, locatedproximal to the distal active pace/sense electrode, by about 0.5 cm to2.5 cm. In this context, bipolar and unipolar sensing may also bereferred to as “near-field” and “far-field” sensing, respectively.(Although “far-field” usually denotes sensing outside the chamber ofinterest, and the unipolar signal derived from such a unipolarpace/sense electrode pair is dominated by the near-field tip electrodesignal.)

[0006] A pacing IPG capable of pacing in atrial synchronized modestypically includes atrial and ventricular sense amplifiers, atrial andventricular pace pulse generators or “amplifiers”, an operating systemgoverning pacing and sensing functions, and components as describedfurther herein in relation to a preferred embodiment of the invention.

[0007] In the typical dual chamber DDD pacing system, an atrial pacing(A-PACE) pulse generated by the atrial pace pulse generator is appliedto the right atrial active and indifferent pace/sense electrodes tocause the right and left atria to depolarize. Similarly, a ventricularpacing (V-PACE) pulse generated by the ventricular pulse generator isapplied to the right ventricular active and indifferent pace/senseelectrodes to cause the right and left ventricles to depolarize. In morerecently developed right and left heart pacing systems, pacing pulsegenerators and leads are incorporated into the pacing system to provideA-PACE and/or V-PACE pulses to the left atrium and/or ventricle.

[0008] The atrial sense amplifier is coupled to atrial active andindifferent pace/sense electrodes to detect electrical signals of theheart associated with atrial depolarizations (P-waves) and to generatean atrial sense event (A-EVENT) signal when detection criteria are met.The ventricular sense amplifier is coupled to ventricular active andindifferent pace/sense electrodes to detect electrical signals of theheart associated with ventricular depolarizations (R-waves) and togenerate a ventricular sense event (V-EVENT) signal when detectioncriteria are met.

[0009] The pacing operating system times out various intervals from eachA-EVENT, V-EVENT, A-PACE, and V-PACE to maintain synchronousdepolarizations of the atria and ventricles. Such AV synchronouspacemakers that perform this function have the capability of trackingthe patient's natural sinus rhythm and preserving the hemodynamiccontribution of the atrial contraction over a wide range of heart rates.Maintenance of AV mechanical synchrony is of great importance as setforth in greater detail in commonly assigned U.S. Pat. No. 5,626,623.

[0010] Typically, the IPG operating system comprises a microcomputercontrolled, digital controller/timer circuit that defines and times outa V-A interval (in DDD and DDDR modes) or a V-V interval (in VDD andVDDR modes) upon a V-EVENT or V-PACE pulse and times out an AV delay inresponse to an A-EVENT (in VDD, VDDR, DDD, DDDR modes) or in response toan A-PACE pulse (in DDD and DDDR modes) as well as a number of otherintervals. An SAV delay is commenced by declaration of an A-EVENT, and aPAV delay is commenced upon delivery of the A-PACE pulse in certain DDDand DDDR pacing systems.

[0011] The A-PACE and V-PACE pulses are produced by the exponentialdischarge of respective atrial and ventricular output capacitors throughthe impedance loads in the atrial and ventricular pacing paths that eachinclude a coupling capacitor, the active and indifferent pace/senseelectrodes, and the patient's heart tissue between the pace/senseelectrodes. In conventional dual chamber pacing systems, both the atrialand ventricular sense amplifiers are “blanked”, i.e., uncoupled, fromthe respective atrial and ventricular pace/sense electrode pairs duringthe delivery of either of an A-PACE pulse or a V-PACE pulse and for aprogrammed blanking period thereafter. The gains of the atrial andventricular sense amplifiers are normally tuned for the relatively lowvoltages of the heart (e.g., 0.3 mV-4.0 mV for the atrial senseamplifier and 1.0 mV-20.0 mV for the ventricular sense amplifier). Thesignificantly greater voltages of the A-PACE and V-PACE pulses (e.g.,varying between 0.5 V and 8.0 V) must be blocked from the atrial andventricular sense amplifiers.

[0012] Moreover, a residual post-pace polarization signal (or“after-potential”) remains in the pacing path due to the residual energyin the impedance load that the output capacitor is discharged into todeliver the A-PACE or V-PACE pulse. The impedance load across the outputamplifier terminals comprises the impedance of the coupling capacitor,the lead conductor(s), the tissue-electrode interface impedances, andthe impedance of the body tissue bulk between the active and indifferentpace/sense electrodes. The impedances of the body tissue and the leadconductor(s) may be modeled as a simple series bulk resistance, leavingthe tissue-electrode interfaces and any coupling capacitors as thereactive energy absorbing/discharging elements of the total load. Thereare typically two tissue-electrode interfaces in a pacing path, one atthe active tip electrode, and one at the indifferent ring (or IPG caseor “can”) electrode. The energy stored in these interfaces and anycoupling capacitors dissipates after the pacing pulse through the pacingpath impedance load creating the after-potential that can be sensed ateach electrode and affect the ability of the sense amplifiers to sensenatural or evoked cardiac events. The tip electrode is the primaryafter-potential storage element in comparison to the case and ringelectrodes. An indifferent ring electrode typically stores more energythan does a can electrode due to differences in electrode areas.

[0013] Most current pacemaker output circuits incorporate “fastrecharge” circuitry for short-circuiting the pacing path and activelydissipating or countering after-potentials during the blanking of thesense amplifier's input terminals to shorten the time that it wouldotherwise take to dissipate after-potentials. The primary purposes ofproviding a recharge operation are to ensure that the couplingcapacitor(s) is recharged to an insignificant voltage level orequilibrium prior to the delivery of the next pacing pulse through itand to allow the net DC current in the pacing path to settle to zero tofacilitate sensing in the same pacing path or using one of thepace/sense electrodes of that pacing path.

[0014] Thus, it is conventional to suppress or blank both of the atrialand ventricular sense amplifiers during A-PACE and V-PACE pulses forblanking periods to avoid overloading the sense amplifier. Moreover, thesense amplifiers may abruptly sense a different potential than waspresent at the time of initial blanking when the blanking period expiresand the sense amplifier is reconnected due to the after-potentials andelectrode polarization as well as the recharge function. This canproduce unwanted oversensing of artifacts resulting in falsedeclarations of A-EVENTs or V-EVENTs. Therefore, the blanking periods inpacemaker IPGs sold by the assignee of this application are nominallyset at 30 ms after delivery of an A-PACE or V-PACE, but the blankingperiods may be programmed as long as 45 ms in difficult sensingscenarios. There may be additional digital blanking of the senseamplifiers to avoid sensing of evoked response or other pacingartifacts, e.g., for 150 ms to 400 ms after paced events in ICDs. Suchblanking periods are characterized as an atrial blanking periods (ABP)including a post atrial pace, atrial blanking period (PAABP or PAAB) anda post ventricular pace, atrial blanking period (PVABP or PVAB) or as aventricular blanking periods (VBP) including a post atrial pace,ventricular blanking period (PAVBP or PAVB), and a post ventricularpace, ventricular blanking period (PVVBP or PVB).

[0015] In addition, a number of sense amplifier refractory periods aretimed out on atrial and ventricular sense event signals and generationof A-PACE and V-PACE pulses, whereby “refractory” A-EVENT and V-EVENTsduring such refractory periods are selectively ignored or employed in avariety of ways to reset or extend time periods being timed out. Atrialand ventricular refractory periods (ARP and VRP) are commenced upon anA-EVENT or V-EVENT or generation of an A-PACE or V-PACE pulse,respectively. The ARP is typically only employed by itself during atrialdemand pacing in the AAI pacing mode. In dual chamber pacing modes, theARP commenced by the A-EVENT or A-PACE pulse extends through the SAVdelay or the PAV delay until a certain time following a V-EVENTterminating the SAV or PAV delay or generation of a V-PACE pulse at theexpiration of the SAV or PAV delay. This post-ventricular atrialrefractory period (PVARP) is commenced by a V-PACE pulse or V-EVENTbased on the understanding that A-EVENTs sensed during its time-outgenerally reflect a retrograde conduction of the evoked or spontaneousventricular depolarization wave and therefore are not employed to resetan escape interval and commence an SAV delay. The duration of PVARP maybe fixed or vary as a function of sensed atrial rate or pacemakerdefined pacing rate, with the result that in many cases relatively longPVARPs are in effect at lower rates. A total ARP (TARP) is defined asthe entire duration of the ARP and the PVARP. See, for example, U.S.Pat. No. 6,311,088. Typically the ARP and VRP are set at 300 ms, and thePVARP durations are programmable in the range of 250 ms-400 ms.

[0016] The rate-adaptive VDDR, DDIR, and DDDR pacing modes function inthe above-described manner but additionally provide rate modulation of apacing escape interval between a programmable lower rate and an upperrate limit (URL) as a function of a physiologic signal or rate controlparameter (RCP) related to the need for cardiac output developed by aphysiologic sensor. At times when the intrinsic atrial rate isinappropriately high or low, a variety of “mode switching” schemes foreffecting switching between tracking modes and non-tracking modes (and avariety of transitional modes) based on the relationship between theatrial rate and the sensor derived pacing rate have been proposed asexemplified by commonly assigned U.S. Pat. No. 5,144,949.

[0017] In order to maximize the useful life of pacing IPGs, it isdesirable that the A-PACE and V-PACE pulse energies be programmed to theminimal energies required to evoke a depolarization of the atria andventricles (i.e., to “capture” the atria and ventricles). The minimumoutput pulse energy which is required to capture and thus evoke amuscular depolarization within the heart is referred to as thestimulation threshold, and generally varies in accordance with the wellknown strength-duration curves, wherein the amplitude of a stimulationthreshold current pulse and its duration are inversely proportional. Onedifficulty that arises from use of the blanking and refractory periodsrelates to the inability to use the sense amplifiers to detect thecapture or loss of capture (LOC) of the atria and ventricles.

[0018] Therefore, it has been proposed to employ additional senseelectrodes and sense amplifiers or differing combinations of pace/senseelectrodes or cardioversion/defibrillation electrodes to sense theevoked response to a V-PACE or A-PACE as described in commonly assignedU.S. Pat. Nos. 5,331,966 and 5,683,431. A subcutaneous electrode array(SEA) formed on the surface of the IPG housing is proposed in the '966patent for sensing the “far field” EGM at a distance from the heartalong vectors selected from the electrodes of the SEA. The far field EGMis employed for a variety of reasons as set forth in theabove-referenced '966 patent. The '966 patent also describes a number ofother sensing schemes in the prior art for sensing the electricalactivity of the heart for determining LOC or other reasons including thefollowing.

[0019] U.S. Pat. No. 3,949,758 relates to a threshold-seeking pacemakerwith automatically adjusted energy levels for pacing pulses in responseto detected LOC, and describes separate sensing and pacing electrodes,which are each utilized in unipolar fashion with a third commonelectrode having a comparatively larger dimension, to reduce residualpolarization problems.

[0020] U.S. Pat. No. 3,977,411 discloses a pacemaker having separatesensing and pacing electrodes that are each utilized in unipolarfashion. The sensing electrode comprises a ring electrode having arelatively large surface area (i.e., between 75 to 200 mm²) for improvedsensing of cardiac activity (R-waves), and is spaced along the pacinglead approximately 5 to 50 mm from the distally-located tip electrodeused for pacing.

[0021] U.S. Pat. No. 3,920,024 discloses a pacemaker having a thresholdtracking capability that dynamically measures the stimulation thresholdby monitoring the presence or absence of an evoked response (R-wave).

[0022] Various electrode configurations are illustrated in FIGS. 1B and9A-9F for purposes of sensing the evoked response, including sensing isbetween an intracardiac electrode and a reference electrode that isspaced some distance away from the heart or sensing between intracardiacelectrodes.

[0023] U.S. Pat. No. 4,305,396 also relates to a rate-adaptive pacemakerwherein the output energy is automatically varied in response to thedetection or non-detection of an evoked response (R-wave) and thedetected stimulation threshold. It is stated to be preferred to use thesame electrode for both pacing and sensing, such as a unipolar orbipolar system wherein there is at least one electrode located in theventricle, but suggests that other lead designs may be utilized suchthat the sensing and pacing electrode are separate.

[0024] U.S. Pat. No. 4,387,717 relates to a pacemaker having a separate(i.e., non-pacing) electrode element, implanted near or in directcontact with the cardiac tissue, and positioned relative to the pacingelectrodes (i.e., unipolar pacing from “tip” to “can”) to provideimproved P-wave and R-wave sensing with minimal interference from thepacing electrodes. The “can” functions as an indifferent electrode forsensing in combination with the separate electrode element. The separatesensing electrode is spaced from the pacing electrodes to minimize crosscoupling and interference from the pacing stimulus and after-potentials.The separate sensing electrode comprises an extravascular metallic platehaving a comparatively large surface area in one embodiment. In anotherembodiment the separate sensing electrode comprises a cylindrical metalring mounted on the insulated pacing lead between the pacemaker and the“tip” electrode, and is described as being located along the lead topermit positioning the sensing electrode either within the heart,externally on the heart wall, or in some remote location in the vascularsystem away from the heart.

[0025] U.S. Pat. No. 4,585,004 relates to an implantable cardiac pacingand monitoring system, wherein the pace/sense electrodes areelectrically separate from an auxiliary sense electrode system. Theauxiliary sense electrode system comprises a transvenous data lead withring electrodes for sensing located in the right ventricle(approximately 1 cm from the pacing tip electrode for R-wave sensing)and in the right atrium (approximately 13 cm from the tip electrode tobe in close proximity with the S-A node), both ring electrodes beingused in conjunction with the pacemaker can in unipolar sensing fashion.

[0026] U.S. Pat. No. 4,686,988 relates to a dual chamber pacemakerhaving atrial and ventricular endocardial leads with a separate proximalring electrode coupled to a P-wave or R-wave sensing EGM amplifier fordetecting the atrial or ventricular evoked response to atrial orventricular stimulation pulses generated and applied to other electrodeson the endocardial lead system. The auxiliary lead system thus resemblesthe '004 patent.

[0027] U.S. Pat. No. 4,549,548 discloses a programmable DDD pacingsystem in which the selection of pace/sense electrodes is changed duringeach pacing cycle to optimize the choice of unipolar and bipolar atrialand ventricular operations. U.S. Pat. Nos. 4,759,366 and 4,858,610relate to evoked response detector circuits that also employ fastrecharge in at least one separate sensing electrode in either unipolaror bipolar electrode configurations in either or both the atrium andventricle. The cardiac pacing systems function as unipolar and bipolarsystems at different steps in the operating cycle. In the '610 patent, aseparate electrode on the connector block of the IPG can is suggestedfor use as the reference electrode anode rather than the metal caseitself if the case is employed as the reference electrode for thedelivery of the stimulation pulse. In the '366 patent, the detectedevoked response is used in an algorithm for adjusting the pacing rate.

[0028] U.S. Pat. Nos. 4,310,000, 4,729,376, and 4,674,508 also disclosethe use of a separate passive sensing reference electrode mounted on theIPG connector block or otherwise insulated from the pacemaker case inorder to provide a sensing reference electrode which is not part of thestimulation reference electrode and thus does not have residualafter-potentials at its surface following delivery of a stimulationpulse. The aforementioned '000 patent suggests various modifications tothe passive sensing reference electrode depicted in its drawings,including the incorporation of more than one passive sensing referenceelectrode provided on or adjacent to the IPG can, positioned as deemednecessary for best sensing, and connected to one or more senseamplifiers. No specific use of the additional passive sensing referenceelectrodes is suggested, although the single passive sensing referenceelectrode is suggested for use with a sense amplifier to detect bothcapture and spontaneous atrial or ventricular electrical events in adual chamber pacing system.

[0029] Moreover, it has been proposed in the prior art to automaticallyselect among pacing and sensing electrode pairs during the cardiac cycleor in response to a determination that lead impedance is unacceptable(which may arise from a lead fracture or electrode dislodgement or thelike). See, for example, U.S. Pat. Nos. 4,958,632, 5,003,975, and5,755,742 and the above-referenced '548 patent. According to the '548patent, the selection of unipolar or bipolar mode of operation is basedon a determination for monitoring the amplitude of sensed heartbeatsignals to determine whether the sensing operation would be performedbetter in the unipolar or the bipolar mode. This is directed to adetermination of heart performance vis-a-vis the leads involved so as tocontrol the selection of unipolar or bipolar sensing.

[0030] Thus, considerable effort has been expended in providing systemsand methods for overcoming the limitations on sensing imposed bydelivery of a pacing pulse across a pair of pace/sense electrodes for avariety of purposes, including detection of LOC and determination ofpacing thresholds, determination of lead impedance, and selection of theoptimal pacing and sensing electrode pairs. Despite these improvements,pacing systems still employ the above-described atrial and ventricularblanking functions.

[0031] Disruption of AV electrical and mechanical synchrony frequentlyarises due to the spontaneous depolarization of the ventricles triggeredat an ectopic site in one of the ventricles. Such a spontaneousventricular depolarization that is not associated with a prior atrialdepolarization is characterized as a premature ventricular contraction(PVC). Many of the problems resulting from the occurrence of a PVC in apatient with a dual chamber pacemaker are described more fully in U.S.Pat. Nos. 4,788,980 and 5,097,832.

[0032] PVCs that occur during the V-A interval following a priordetected R-wave or delivery of a V-PACE pulse are usually sensed asV-EVENTs that restart the V-A interval. PVCs that occur during thetime-out of the AV delay and following time-out of the PAVBP areindistinguishable from sinus ventricular depolarizations that areconducted from the AV node through the Bundle of His. The resultingV-EVENT inhibits delivery of the V-PACE, and the V-A interval iscommenced.

[0033] As noted above, after-potentials on the ventricular pace/senseelectrodes at time-out of the PAVBP can erroneously be detected andresult in declaration of a V-EVENT by the ventricular sense amplifier.The pacing system will not provide appropriate ventricular pacing to apatient's heart having AV block if electrical noise or other signals aremistakenly sensed by the ventricular sense amplifier as V-EVENTS duringtime-out of the AV delay.

[0034] The questionable nature and consequences of mistakenly detectingV-EVENTs has led to the adoption of the practice of delivering aventricular safety pace (VSP) pulse at a fixed time, typically 110 ms,following delivery of an A-PACE. In other words, a VSP pulse isdelivered to the ventricular pace/sense electrodes if a V-EVENT isdeclared between the time-out of the PAVBP and a 110 ms VSP windowfollowing delivery of an A-PACE pulse.

[0035] This 110 ms VSP window is often denoted the cross talk window.The 110 ms VSP window length is shorter than the normal AV conductiontime in humans, so any V-EVENT declared within the VSP window isunlikely to be due to true AV conduction. The delivered VSP pulsecaptures the ventricles if the V-EVENT was due to cross talk, that is,sensing of the residual A-PACE energy afterpotentials. The delivered VSPpulse will not capture the ventricles if the V-EVENT reflects a PVC,because the ventricles will be refractory at that time. Thus, faced withthis uncertainty, a VSP pulse is delivered at time-out of the VSP windowor delay so as to ensure that the ventricles are truly contracting at asafe time after delivery of the A-PACE pulse. The VSP function is aprogrammable feature of prior art pacing systems that may be programmedoff by the physician if desired. One form of VSP operation is set forthin U.S. Pat. No. 4,825,870, for example.

[0036] However, it frequently happens that the depolarization wavefrontof a PVC reaches the pace/sense electrodes during the PAVBP, and theventricular sense amplifier does not detect the R-wave. Theafter-potentials from the PVC wavefront may not be strong enough at theventricular pace/sense electrodes to trigger a V-EVENT at time-out ofthe ventricular blanking period. Thus, a V-PACE pulse may be deliveredat the time-out of the AV delay. The AV delay may be programmed to belong enough so that the V-PACE is delivered during the vulnerable periodof the ventricles. The vulnerable period occurs during the T-waverepolarization of the ventricle (approx. 250 ms-400 ms). During thevulnerable period, there is a dispersion of refractoriness where somecardiac cells are repolarized while others are still refractory.Additional stimulation during this time has a higher likelihood ofinitiating a tachyarrhythmia than during periods where the cardiac cellsare either completely refractory or completely repolarized.

BRIEF SUMMARY OF THE INVENTION

[0037] In accordance with the present invention, AV synchronous, dualchamber pacing systems or any atrial based pacing system requiringventricular sensing are provided having improved sensing of normalventricular depolarizations or ectopic ventricular depolarizationscoincidentally occurring at or shortly following delivery of an A-PACEpulse. Ventricular activations can occur coincident with an A-PACE pulseor otherwise within the PAVBP in a number of scenarios, such as ectopicventricular depolarizations, also referred to as premature ventricularcontractions (PVCs) and normal ventricular activations during atrialunder-sensing or intermittent loss of atrial capture. For convenienceand because the most common form of under-sensed ventricular activationis due to PVCs, any such ventricular depolarization occurring coincidentwith the delivery of an A-PACE pulse is characterized herein as a PVC.

[0038] The QRS complex of such a PVC that appears between tightlyspaced, near field, ventricular pace/sense electrodes is relativelynarrow and exhibits a pronounced R-wave peak that is excellent forventricular sensing when the ventricular sense amplifier is not blanked.Accordingly, the ventricular sense amplifier is preferably coupled withbipolar pace/sense electrodes and advantageously provides robust sensingof PVCs or conducted R-waves when it is not blanked. However, the narrowQRS complex sensed across the closely spaced ventricular pace/senseelectrodes dissipates by the time that the PAVBP times-out as thedepolarization wave front propagates through the ventricles and past theventricular pace/sense electrodes. Therefore, the R-wave peak of a PVCoccurring within the PAVBP is not sensed by the ventricular senseamplifier when the PAVBP times-out. A need therefore remains for acapability of sensing such PVCs falling within the PAVBP

[0039] We have observed that the QRS complexes of such PVCs observedacross widely spaced sense electrodes are relatively wide and are lesssusceptible to under-sensing during the PAVBP. We have also observedthat sense electrodes that are spatially separated from the ventricularpace/sense electrodes add additional sensing capabilities because of thepropagation delay of the QRS wavefront between such remote electrodes.In accordance with the present invention, a PVC occurring coincidentwith or shortly following delivery of an A-PACE pulse that would fallwithin the PAVBP is sensed employing a PVC sense amplifier that iscoupled to such widely spaced sense electrodes that do not include bothof the ventricular pace/sense electrodes coupled to the ventricularsense amplifier subjected to the PAVBP. The PVC sense amplifier may beblanked simply during delivery of the A-PACE pulse to protect the senseamplifier circuitry from the applied pacing voltage, but can then sensethe relatively wide QRS complex of the PVC that persists longer than theA-PACE pulse.

[0040] Therefore, in one embodiment of the present invention, a firstventricular sense amplifier is coupled to active and indifferentventricular pace/sense electrodes for sensing natural ventriculardepolarizations and declaring a V-EVENT. The first ventricular senseamplifier is blanked during the PAVBP following delivery of an A-PACEpulse. A far field or unipolar PVC sense amplifier coupled to a farfield, PVC sense electrode pair detects such PVCs while the ventricularsense amplifier coupled to the active and indifferent ventricularpace/sense electrodes is blanked. The far field PVC sense electrode pairis disposed in the patient's body to define a far field PVC sense vectordiffering from a ventricular sense vector defined by the active andindifferent ventricular pace/sense electrodes.

[0041] In another aspect of the present invention, the VSP function isadvantageously augmented by the redundant sensing capability provided bythe first ventricular sense amplifier and the PVC sense amplifier. Asdescribed above, when a PVC is under-sensed in a dual chamber pacingsystem, a V-PACE pulse is delivered at the end of the AV interval. Atnominal AV intervals, the ventricle is typically refractory to asubsequent V-PACE pulse. However, a V-PACE pulse delivered after a longAV interval has a greater probability of capturing the heart. The V-PACEpulse may be delivered within a patient's vulnerable period and incertain circumstances may initiate an arrhythmia in a susceptiblepatient. The mounting evidence suggesting long-term deleterious effectsof right ventricular apical pacing may increase physician motivation toextend the AV interval to decrease ventricular pacing. Ventricularsafety pacing ensures a ventricular beat for each cardiac cycle andensures that the V-PACE pulse is not delivered in the ventricularvulnerable period. This is accomplished by delivering the VSP pulseshortly after the A-PACE pulse when a V-EVENT is detected closelyfollowing the delivery of the A-PACE pulse. The subsequent VSP pulsewill capture the heart if a V-EVENT was declared due to noise, but thesubsequent VSP pulse will not capture the heart if the V-EVENT was dueto sensing of a PVC. In accordance with this aspect of the presentinvention, a VSP pulse is delivered if either of the ventricular senseamplifier that is subjected to the PAVBP or the PVC sense amplifierdeclares a V-EVENT. In this way, the sensing of such PVCs occurringcoincident with the delivery of A-PACE pulses is improved and thepotential for ventricular pacing during the vulnerable period isminimized.

[0042] In the simplest atrial pacing systems, the PVC sense electrodepair can comprise one of the ventricular pace/sense electrodes and anindifferent electrode supported on or comprising the conductive IPG candefining a unipolar PVC sense vector. Or, the PVC sense electrode paircan comprise a selected pair of sense electrodes of an SEA supported bythe IPG enclosure defining an optimal PVC sense vector. Or, in an ICDcontext providing atrial pacing, the PVC sense electrode pair cancomprise a further cardioversion/defibrillation electrode pair definingan optimal PVC sense vector or can comprise one of the furthercardioversion/defibrillation electrodes and the indifferent electrodesupported on or comprising the conductive IPG can defining a optimal PVCsense vector. Or, in a right and left heart pacing context providingatrial pacing, the PVC sense electrode pair can comprise right and leftheart chamber pace/sense electrodes defining an optimal PVC sense vectoror can comprise one of the left heart chamber pace/sense electrodes andthe indifferent electrode supported on or comprising the conductive IPGcan defining an optimal PVC sense vector.

[0043] Preferably, the far field sense electrode pair can be selected ina test routine or work-up by the physician commenced by programming aPVC sense electrode pair coupled with the PVC sense amplifier andentering a test routine. The results of the test routines of availablePVC sense electrode pairs can be compared to identify the optimal PVCsense vector.

[0044] As noted above, the ability to detect a PVC during the PAVBP canbe employed advantageously to trigger VSP pacing or to inhibitventricular pacing, which in either case avoids delivery of a V-PACEpulse at the time-out of the PAV delay possibly into the vulnerableperiod of the heart cycle. The ability to detect a PVC at other timesduring the PAV or SAV delay or the V-A interval can advantageously beemployed to confirm declarations of V-EVENTs, leading to more robustV-EVENT sensing.

[0045] Advantageously, the PVC sense amplifier can be enabled during thecardiac cycle, to function as a conventional EGM sense amplifier so thatthe spontaneously occurring PQRST complexes can be recorded for realtime analysis or data storage as is well known in the art.

[0046] This summary of the invention has been presented here simply topoint out some of the ways that the invention overcomes difficultiespresented in the prior art and to distinguish the invention from theprior art and is not intended to operate in any manner as a limitationon the interpretation of claims that are presented initially in thepatent application and that are ultimately granted.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] These and other advantages and features of the present inventionwill be more readily understood from the following detailed descriptionof the preferred embodiments thereof, when considered in conjunctionwith the drawings, in which like reference numerals indicate identicalstructures throughout the several views, and wherein:

[0048]FIG. 1 is a schematic illustration of a dual chamber pacemakerimplanted in a patient's chest comprising an IPG and endocardial leadstransvenously introduced into the right atrium and right ventricle ofthe heart, wherein PVC sensing can be conducted during the PAVBP acrossselected far field sensing electrode pairs;

[0049]FIG. 2 is a block diagram of the pacing IPG of FIG. 1 in which thepresent invention may be practiced;

[0050]FIG. 3 is flow chart depicting the steps of a DDD pacing cycle;

[0051]FIG. 4 is a detailed flow chart depicting the steps of detectingand responding to a PVC sensed during the time-out of the PAVBP;

[0052]FIG. 5 is a schematic illustration of a further embodiment of adual chamber pacemaker implanted in a patient's chest comprising an IPGsupporting a SEA and endocardial leads transvenously introduced into theright atrium and right ventricle of the heart, wherein PVC sensing canbe conducted during the PAVBP across selected far field SEA senseelectrode pairs;

[0053]FIG. 6 is a block diagram of the pacing IPG of FIG. 5 in which thepresent invention may be practiced;

[0054]FIG. 7 is a schematic illustration of a further embodiment of adual chamber, right and left heart pacemaker implanted in a patient'schest and endocardial leads transvenously introduced into the rightatrium, right ventricle and coronary sinus of the heart, wherein PVCsensing can be conducted during the PAVBP across selected right and leftheart sense electrode pairs;

[0055]FIG. 8 is a block diagram of the pacing IPG of FIG. 7 in which thepresent invention may be practiced;

[0056]FIG. 9 is a schematic illustration of a dual chamber pacing ICDimplanted in a patient's chest comprising an IPG and endocardial leadstransvenously introduced into the right atrium, right ventricle, andcoronary sinus of the heart supporting pace/sense and/orcardioversion/defibrillation electrodes, wherein PVC sensing can beconducted during the PAVBP across selected far field sensing electrodepairs; and

[0057]FIG. 10 is a block diagram of the ICD IPG of FIG. 7 in which thepresent invention may be practiced.

DETAILED DESCRIPTION OF THE INVENTION

[0058] In the following detailed description, references are made toillustrative embodiments of methods and apparatus for carrying out theinvention. It is understood that other embodiments can be utilizedwithout departing from the scope of the invention.

[0059]FIGS. 1 and 2 depict the external configuration and components ofa typical implantable dual chamber pacemaker operating in a DDD, DDI,DDIR, or DDDR pacing mode or operating in an MI or MIR pacing mode toprovide atrial pacing in the absence of an adequate atrial heart rate aslong as ventricular sensing indicates normal AV conduction. Such a dualchamber IPG 100 and unipolar or bipolar atrial and ventricular leads 114and 116 (bipolar leads are depicted), in which the present invention maybe implemented is depicted in FIGS. land 2. The dual chamber pacemakerIPG 100 senses and paces in the atrial and ventricular chambers, andpacing is either triggered and inhibited depending upon sensing ofintrinsic, non-refractory atrial and ventricular depolarizations duringthe sequentially timed V-A interval and AV delay, respectively, as iswell known in the art, in accordance with the steps set forth in theflow chart of FIG. 3. The present invention functions when the atria arepaced due to failure to detect atrial depolarizations or when the sensedatrial heart rate falls below a rate dictated by a RCP related to theneed for cardiac output developed by a physiologic sensor.

[0060] In addition, the present invention can be implemented in such adual chamber pacing system that is incorporated into a dual chamberpacing ICD or into a right and left heart pacing system by itself orthat is incorporated into a multi-chamber pacing IPG. The followingdescription is thus intended to encompass all of the various types ofdual chamber pacemaker systems in which the present invention can beimplemented.

[0061] The IPG 100 is provided with a hermetically sealed enclosure orcan 118, typically fabricated of bio-compatible metal such as titanium,enclosing the dual chamber IPG circuit 300 depicted in FIG. 2. Aconnector block assembly 112 is mounted to the top of the can 118 toreceive electrical connectors located on the proximal connector ends ofthe depicted bipolar atrial and ventricular pacing leads 114 and 116.

[0062] As described further below, an electrically exposed area of thecan 118 functions as an IND_CAN electrode 140 that is electricallyconnected to one input of a PVC sense amplifier to facilitate sensing ofPVCs over the heart cycle, particularly to facilitate sensing PVCsduring the PAVBP following delivery of an A-PACE pulse.

[0063] The bipolar atrial pacing lead 116 extends between its proximalconnector coupled to IPG 100 and distal atrial pace/sense electrodes 120and 122 located in the right atrium 12 of heart 10 to enable sensing ofP-waves and delivery of atrial pacing pulses to the right atria. Atrialpacing pulses may be delivered between electrodes 120 and 122 in abipolar pacing mode or between electrode 122 and the IND_CAN electrode140 of the IPG 100 in a unipolar pacing mode. Sensing of P-waves by theatrial sense amplifier subject to atrial blanking may occur betweenelectrode 120 and electrode 122 in a bipolar sensing mode or betweeneither of electrode 120 and 122 and the IND_CAN electrode 140 of the IPG100 in a unipolar atrial sensing mode.

[0064] Similarly, the bipolar ventricular pacing lead 114 extendsbetween its proximal connector coupled to IPG 100 and distal ventricularpace/sense electrodes 128 and 130 located in the right ventricle 16 ofheart 10 to both sense R-waves and to deliver ventricular pacing pulsesto the ventricles. Ventricular pacing pulses may be delivered betweenelectrodes 128 and 130 in a bipolar pacing mode or between electrode 130and the IND_CAN electrode 140 of the IPG 100 in a unipolar pacing mode.Sensing of R-waves by the ventricular sense amplifier subject toblanking occurs between electrodes 128 and 130 in a bipolar sensing modein this preferred embodiment.

[0065] The IPG circuit 300 within IPG 100 and the bipolar atrial andventricular leads 114 and 116 are depicted in FIG. 2 in relation toheart 10. The IPG circuit 300 is divided generally into a microcomputercircuit 302 and a pacing input/output circuit 320. The input/outputcircuit 320 includes the digital controller/timer circuit 330, theatrial and ventricular pacing pulse output circuit 340 and the atrialand ventricular sense amplifiers circuit 360, as well as a number ofother components and circuits described below. The digitalcontroller/timer circuit 330 provides control of timing and otherfunctions within the input/output circuit 320. Digital controller/timercircuit 330, operating under the general control of the microcomputercircuit 302, includes a set of timing and associated logic circuits, ofwhich certain ones pertinent to the present invention are depicted anddescribed further below.

[0066] Preferably, the IPG 100 or one of the leads 114 or 116 includesone or more physiologic sensor that develops a physiologic signal thatrelates to the need for cardiac output. The use of physiologic sensorsto provide variation of pacing rate in response to sensed physiologicparameters, such as physical activity, oxygen saturation, blood pressureand respiration, has become commonplace.

[0067] Commonly assigned U.S. Pat. Nos. 4,428,378 and 4,890,617 discloseactivity sensors that are employed to vary the pacing escape interval insingle and dual chamber pacemaker IPGs in response to sensed physicalactivity. Such an activity sensor 316 is coupled to the inside surfaceof the IPG hermetically sealed enclosure 118 and may take the form of apiezoelectric crystal transducer as is well known in the art. Theactivity sensor 316 generates an output signal in response to certainpatient activities, e.g. ambulating, that is processed and used as arate control parameter (RCP). If the IPG operating mode is programmed toa rate responsive mode, the patient's activity level developed in thepatient activity circuit (PAS) 322 is monitored, and a sensor derivedV-A, A-A or V-V escape interval is derived proportionally thereto. Atimed interrupt, e.g., every two seconds, may be provided in order toallow the microprocessor 304 to analyze the output of the activitycircuit PAS 322 and update the basic V-A (or A-A or V-V) escape intervalemployed to govern the pacing cycle and to adjust other time intervalsas described below.

[0068] The bipolar leads 114 and 116 are illustrated schematically withtheir associated pace/sense electrode sets 120, 122 and 128, 130,respectively, as coupled directly to the atrial and ventricular pacingpulse output circuit 340 and sense amplifiers circuit 360 of pacingcircuit 320. The atrial and ventricular pacing pulse output circuit 340and sense amplifiers circuit 360 contain pulse generators and senseamplifiers corresponding to any of those presently employed incommercially marketed cardiac pacemakers for atrial and ventricularpacing and sensing.

[0069] Sense amplifiers circuit 360 also comprises a PVC sense amplifiercoupled with the IND_CAN electrode 140 and one of the ventricularpace/sense electrodes 128 or 130 selected by a ventricular select(V-SELECT) signal so that PVCs can be sensed along a bipolar orfar-field sense vector. It will be understood that other senseelectrodes can be coupled to the PVC sense amplifier within the senseamplifiers circuit 360 and selected by an appropriate V-SELECT signalthrough programming commands in the course of a telemetry session.

[0070] Sensitivity settings of the atrial and ventricular senseamplifiers and the PVC sense amplifier in sense amplifiers circuit 360can be programmed by the physician to reliably sense true P-waves,R-waves and PVCs during a patient work-up at implantation or during apatient follow-up telemetry session. Digital controller/timer circuit330 controls the sensitivity settings of the atrial and ventricularsense amplifiers in sense amplifiers circuit 360 by means of sensitivitycontrol 350.

[0071] The depicted counters and timers within digital controller/timercircuit 330 include ABP and VBP timers 366, intrinsic interval timers368 for timing average intrinsic A-A and V-V intervals from A-EVENTs andV-EVENTs, escape interval timers 370 for timing A-A, V-A, and/or V-Vpacing escape intervals, an AV delay timer 372 for timing the SAV delayfrom a preceding A-EVENT or PAV delay from a preceding A-TRIG,refractory period timers 374 for timing ARP, PVARP and VRP times and aPVC flag register 376 that is set upon detection of a PVC. Digitalcontroller/timer circuit 330 starts and times out these intervals andtime periods that are calculated by microcomputer circuit 302 forcontrolling the above-described operations of the atrial and ventricularsense amplifiers in sense amplifiers circuit 360 and the atrial andventricular pace pulse generators in output amplifier circuit 340.

[0072] In order to trigger generation of a V-PACE pulse, digitalcontroller/timer circuit 330 generates a V-TRIG signal at the end of aPAV or SAV delay provided by AV delay timer 372. Similarly, in order totrigger an atrial pacing or A-PACE pulse, digital controller/timercircuit 330 generates an A-TRIG signal at the termination of the V-Ainterval timed out by escape interval timers 370.

[0073] The ABP and VBP timers 366 of digital controller/timer circuit330 time out the above-described PAVBP and PAABP during and following anA-PACE pulse and the PAVBP and PVVBP during and following a V-PACEpulse. Thus, an atrial blanking (A-BLANK) signal is applied to theatrial sense amplifier for the prevailing ABP, and a ventricularblanking (V-BLANK) signal is applied to the ventricular sense amplifierfor the prevailing VBP. In the absence of an A-BLANK signal, atrialdepolarizations or P-waves that are detected by the atrial senseamplifier result in an A-EVENT that is communicated to the digitalcontroller/timer circuit 330. Similarly, in the absence of a V-BLANKsignal, ventricular depolarizations or R-waves that are detected by theventricular sense amplifier result in a V-EVENT that is communicated tothe digital controller/timer circuit 330. In accordance with the presentinvention, the PVC sense amplifier within sense amplifiers circuit 360is only blanked during delivery of the A-PACE pulse to prevent thedelivered A-PACE pulse from either damaging the sense amplifiercircuitry or being incorrectly sensed as a PVC.

[0074] The refractory period timers 374 time the ARP from an A-TRIGpulse or A-EVENT during which a sensed A-EVENT is ignored for thepurpose of resetting the V-A interval. The ARP extends from thebeginning of the SAV or PAV interval following either an A-EVENT or anA-TRIG and until a predetermined time following a V-EVENT or a V-TRIG.The refractory period timers 374 also time the PVARP from a V-TRIG pulseor V-EVENT during which a sensed A-EVENT is also ignored for the purposeof resetting the V-A interval. The VRP is also be timed out by therefractory period timers 374 after a V-EVENT or V-TRIG signal so that asubsequent, closely following V-EVENT is ignored for the purpose ofrestarting the V-A interval and setting the PVC flag in register 366.

[0075] The base ARP, PVARP and VRP that prevails at the lower rate of60-70 bpm, for example, are either default or programmed parametervalues stored in the microcomputer 302. These refractory periodparameter values can be fixed for the full operating range of pacingrates between the programmed lower rate and the URL, which may be 120bpm, for example, or they can be programmed to follow the algorithm forautomatically shortening in duration as the paced or intrinsic heartrate increases to ensure that the long refractory periods during thediminishing escape intervals do not prevent delivery of ventricularpacing pulses synchronized to valid intrinsic P-waves.

[0076] The A-EVENT is characterized as a refractory A-EVENT if it occursduring time-out of an ARP or a PVARP or a non-refractory A-EVENT if itoccurs after time-out of these atrial refractory periods. Similarly, aV-EVENT is characterized as a refractory V-EVENT if it occurs duringtime-out of a VRP or a non-refractory V-EVENT if it occurs aftertime-out of the ventricular refractory period. Refractory A-EVENTs andV-EVENTs are typically ignored for purposes of resetting timed out AVdelays and V-A intervals, although diagnostic data may be accumulatedrelated to their occurrences.

[0077] Microcomputer 202 contains a microprocessor 304 and associatedsystem clock 308 and on-processor RAM and ROM chips 310 and 312,respectively. In addition, microcomputer circuit 302 includes a separateRAM/ROM chip 314 to provide firmware and additional RAM memory capacity.Microprocessor 304 is interrupt driven, operating in a reduced powerconsumption mode normally, and awakened in response to defined interruptevents, which may include the A-TRIG, V-TRIG, A-EVENT and V-EVENTs.

[0078] Microcomputer 302 controls the operational functions of digitalcontroller/timer 324, specifying which timing intervals are employed ina programmed pacing mode via data and control bus 306. The specificvalues of the intervals timed by the digital controller/timer circuit330 are controlled by the microcomputer circuit 302 by means of data andcontrol bus 306 from programmed-in parameter values. The microcomputer302 also calculates the RCP derived or intrinsic atrial rate derivedV-V, A-A or V-A interval, the variable AV delay, and the variable ARP,PVARP and VRP. Typically, the AV delay in modern VDD, VDDR, DDD and DDDRpacemakers is either fixed or varies with the prevailing intrinsicatrial rate, measured as an A-A interval, and/or varies as a function ofa physiologic sensor derived pacing rate.

[0079] Digital controller/timer circuit 330 also interfaces with othercircuits of the input output circuit 320 or other components of IPGcircuit 300. Crystal oscillator circuit 338 provides the basic timingclock and battery 318 provides power for the pacing circuit 320 and themicrocomputer circuit 302. Power-on-reset circuit 336 responds toinitial connection of the circuit to the battery 318 for defining aninitial operating condition and similarly, resets the operative state ofthe IPG circuit 300 in response to detection of a low battery condition.Reference mode circuit 326 generates stable voltage reference andcurrents for the analog circuits within the pacing circuit 320. ADC(analog to digital converter) and multiplexer circuit 328 digitizesanalog signals and voltage to provide real time telemetry of cardiacsignals from sense amplifiers 360, for uplink transmission via RFtransmitter and receiver circuit 332. Voltage reference and bias circuit326, ADC and multiplexer 328, power-on-reset circuit 336 and crystaloscillator circuit 338 may correspond to any of those presently used incurrent marketed implantable cardiac pacemakers.

[0080] Data transmission to and from an external programmer (not shown)during a telemetry session is accomplished by means of the telemetryantenna 334 and an associated RF transmitter and receiver 332, whichserves both to demodulate received downlink telemetry and to transmituplink telemetry. Uplink telemetry capabilities will typically includethe ability to transmit stored digital information, e.g. operating modesand parameters, EGM histograms, and other events, as well as real timeEGMs of atrial and/or ventricular electrical activity and Marker Channelpulses indicating the occurrence of sensed and paced depolarizations inthe atrium and ventricle, as are well known in the pacing art.

[0081] Reed switch 317 when closed by application of a magnetic fieldmay be employed to enable programming of the pacemaker and also may beemployed to convert the pacemaker temporarily to an asynchronous pacingmode such as DOO or VOO. Operation in the asynchronous mode may continueas long as the magnetic field is present, may continue until overriddenby the programmer or may continue for a pre-set time period.

[0082] The illustrated IPG circuit 300 of FIG. 2 is merely exemplary,and corresponds to the general functional organization of mostmulti-programmable microprocessor controlled DDD and DDDR cardiacpacemaker IPGs presently commercially available. It is believed that thepresent invention can readily be practiced using the basic hardware andsoftware of existing microprocessor controlled, dual chamber pacingsystems that are incorporated into dual chamber pacemakers or into ICDsor into right and left heart pacing systems. The invention is preferablyimplemented into the exemplary pacing system by means of modificationsto the hardware incorporating the PVC sense amplifier to detect signalsacross a PVC sense vector during the PAVBP and at other times during thepacing cycle and to declare a PVC if the signal (regardless of its truesource) satisfies PVC detection criterion. In addition, software storedin the ROM 310 of the microcomputer circuit 302 responding to suchdetected PVCs is modified as described further below. However, theoperating functions of the present invention may also be usefullypracticed by means of a full custom integrated circuit, for example, acircuit taking the form of a state machine, in which a state counterserves to control an arithmetic logic unit to perform calculationsaccording to a prescribed sequence of counter controlled steps.

[0083]FIG. 3 is a functional flow chart of the overall pacing cycletiming operation of the pacemaker IPG circuit 300 illustrated in FIG. 2in the DDD or DDDR pacing modes. In the flow chart of FIG. 3, it isassumed that the A-A or V-V escape interval, cardiac cycle timing of theIPG circuit 300 ranges between a programmed lower rate and a programmedURL and is based on the definition of a V-A interval and an AV delay,specifically either the SAV or the PAV delay interval. The AV delay andV-A interval of any given pacing cycle may be determined as a functionof a sensor-derived V-A interval or an atrial rate based V-A intervaldetermined by the average measured intrinsic A-A atrial rate if it isstable and varies between the programmed lower rate and URL. In thisparticular embodiment, separate SAV and PAV delays are defined, althoughin practice they may have the same duration. The operations of the flowchart may also incorporate any of the mode switching and sinuspreference algorithms of the prior art described above to switch betweenthe use of the sensor or the atrial rate derived escape intervals.However the algorithm is specifically implemented, it is understood toincorporate the PVC response algorithm of the present invention asdescribed hereafter.

[0084] For convenience, the pacing cycle is assumed to begin at stepS100 starting from a non-refractory A-EVENT. Timing of the prevailingSAV delay and ARP are commenced in step SI 00, and the system awaitseither time out of the SAV delay in step S102 or a non-refractoryV-EVENT in step S104.

[0085] Neither of the atrial and ventricular sense amplifiers isblanked, and the PVC sense amplifier may also be enabled. A V-TRIG andthe associated A-BLANK and V-BLANK signals are generated at step S106 atthe end of the SAV delay if a non-refractory V-EVENT does not occur atstep S104 prior to SAV time-out in step S102.

[0086] The SAV delay is terminated without delivery of a V-PACE pulse ifeither of a PVC or a V-EVENT is declared or if both a PVC and a V-EVENTare declared in step S124, and the V-A interval is restarted in stepS108. The redundant sensing of PVCs or other signals by the PVC senseamplifier and the near field R-wave sense amplifier during time-out ofthe SAV delay provides a robust sensing capability that increasesconfidence that unnecessary pacing of the ventricles is avoided.

[0087] The V-A interval time-out is commenced in step S108, and time-outof the post ventricular time periods including the VRP, PVARP, PAVBP andPWBP are commenced in step S110. The algorithm awaits expiration of theV-A interval at step S112, and it is possible that a refractory ornon-refractory A-EVENT or V-EVENT can occur during the V-A intervaltime-out.

[0088] If a non-refractory A-EVENT is sensed in step S120 duringtime-out the V-A interval, the V-A interval is terminated, the AV delayis set to the SAV delay in step S124, and the SAV delay and associatedpost atrial sense ARP is timed out in step S100. Optionally, thenon-refractory A-EVENT also causes the V-A interval to be measured byintrinsic interval timer 368 and employed to derive or update theintrinsic atrial rate that is saved in RAM. The V-A interval, the SAVand PAV delays, the PVARP, and the pacing escape interval for the nextcardiac cycle can then be recalculated in dependence upon either theupdated average A-A interval or upon the RCP in a manner well known inthe art.

[0089] If a non-refractory V-EVENT is declared sensed by the near fieldor bipolar ventricular sense amplifier at step S122 during time out ofthe V-A interval in the absence of detection of a preceding A-EVENT,then the declared V-EVENT is characterized as a PVC in step S124. Itshould be noted that such a declaration of a V-EVENT during the V-Ainterval can be confirmed by the declaration of a PVC by the PVC senseamplifier. Certain algorithms, e.g., those disclosed in theabove-referenced '088 patent, have been devised to deal with such PVCsoccurring during the V-A interval that could be practiced along with butare not necessary to the practice of the present invention.

[0090] An A-TRIG signal is generated in step S114 at the time-out of theV-A interval if the V-A interval times out without sensing any suchintervening non-refractory A-EVENT or V-EVENT. In this case, the nextsucceeding AV delay is defined to be equal to PAV at step S116, and thePAV is timed out in step S118 along with the associated VSP delay andthe ARP, ABP and PAVBP in accordance with the steps of FIG. 4. Theparticular algorithm of FIG. 4 assumes that a VSP function is providedand that the VSP delay is timed out in timers 366 whenever a V-PACE isdelivered, but the present invention can be practiced without the VSPfunction being present or programmed on in a particular case. Moreover,the algorithm of FIG. 4 assumes that the PVC sense amplifier is alwaysenabled, but the PVC sense amplifier could be blanked or disabled duringdelivery of the A-PACE pulse and V-PACE pulse.

[0091] The time-out of the PAV delay is monitored in step S128, and aV-PACE pulse is delivered in step S138 if the PAV delay does time-outwithout declaration of either of a PVC or a V-EVENT. In step S130, a PVCcan be declared at any time during the PAV delay and a V-EVENT can bedeclared following the time-out of the PAVBP. If the VSP function is notpresent or programmed ON as determined in step S132, then such adeclared PVC or V-EVENT would simply cause the V-A interval to commencein step S108.

[0092] However, preferably the VSP function is employed as determined instep S132, and a declared PVC or V-EVENT causes the V-A interval tocommence in step S108 only if it is declared after time-out of the VSPdelay. If a PVC or V-EVENT is declared in step S130 before time-out ofthe VSP delay, then a V-PACE is delivered in step S140 at time-out ofthe VSP delay.

[0093] To enable this function, a VSP flag is set in step S136 if a PVCor V-EVENT is declared in step S130 before time-out of the VSP delay asdetermined in step S134. The status of the VSP flag is checked in stepS138 when the VSP delay does time-out as determined in step S134. Sincethe VSP flag was set in this instance in step S136, then the V-PACEpulse is delivered at time-out of the VSP delay. In this way, a PVC thatwould otherwise not be detected during the PAVBP does trigger the VSPfunction to pace the ventricles within a safe time from the PVC and notwithin the vulnerable period of the heart.

[0094] If a PVC or V-EVENT is declared in step S130 after time-out ofthe VSP delay, as determined in step S134, then a V-PACE is notdelivered in step S140. The time-out of the PAV delay is terminated, andthe V-A interval is started in step S108. The redundant sensing of PVCsor other signals by the PVC sense amplifier and the near field R-wavesense amplifier in the time period between the end of the VSP delay andthe time-out of the PAV delay provides a robust sensing capability thatincreases confidence that unnecessary pacing of the ventricles isavoided.

[0095] The PVC sense amplifier of the depicted embodiment of FIGS. 1 and2 senses the far field R-wave to particularly detect PVCs across thesensing vector comprising the IND_CAN electrode 10 and one of the ringand tip ventricular pace/sense electrodes 128 and 130. It is expectedthat the PVC sense amplifier could be advantageously coupled to theIND_CAN electrode 10 and the ring pace/sense electrode 128 because itmay not be in the blood and not in contact with endocardial surfaceresulting in a wide QRS complex. It will be understood from thefollowing that other far field sensing vectors can be selected dependingon the available sensing electrodes of the pacing orcardioversion/defibrillation system. The PVC sense amplifier sensitivitycan be programmed in a telemetry session to sense intrinsic R-wavesappearing in a conventional ECG display. The PVC sense amplifier'suplink telemetered response (the presence or absence of a PVC outputsignal) can be observed simultaneously. The PVC sense amplifiersensitivity can be varied for each programmed PVC sense vector, and thePVC sense vector providing the best consistent detection of R-waves canbe determined. A “permanent” V-SELECT can then be programmed forcoupling the PVC sense amplifier inputs to receive the optimal pair ofsignals across the PVC sense electrodes during chronic implantation.

[0096] The present invention including the steps of FIGS. 3 and 4, canbe practiced in a dual chamber pacemaker of the type depicted in FIGS. 5and 6 comprising an IPG 100′ supporting a SEA on the IPG housingcomprising at least one pair of sense electrodes whereby a sense vectoror sense vectors can be defined between the sense electrodes. The IPG100′ and IPG circuit 300′ conform in most ways to the IPG 100 and IPGcircuit 300 described above in reference to FIGS. 1 and 2 with theaddition of the SEA. Preferably, the SEA comprises at least three orfour orthogonally disposed sense electrodes or more than four senseelectrodes disposed around the IPG housing including the IPG connectorblock and the hermetically sealed enclosure. In the depicted example,the SEA comprises sense electrodes 142, 144, and 146 with senseelectrode 146 disposed either on the IPG connector block 112′ or the IPGhermetically sealed housing 118′. As in the embodiment of FIGS. 1 and 2,endocardial leads 114 and 116 transvenously introduced into the rightatrium 12 and right ventricle 16 of the heart 10. The IPG circuit 300′can select the optimal sensing vector sensed by the PVC sense amplifierwithin a sense amplifiers circuit 360′ by an appropriate V-SELECTcommand operating additional PVC sense amplifier input switchingcircuitry of the type disclosed in the above-referenced '966 patent.

[0097] The sense electrodes 142, 144, and 146 or the SEA are situated onthe IPG housing comprising the connector block 112′ and/or thehermetically sealed enclosure 118′ so to at least four sense vectorsthat are characterized as far field sense vectors because the SEA islocated subcutaneously remote from the heart 10. The SEA provides threeor four far field PVC sense vectors comprising PVC sense vector A-Bbetween sense electrodes 146 and 144, PVC sense vector B-C between senseelectrodes 144 and 142, and PVC sense vector A-C between senseelectrodes 142 and 146 by appropriately coupling the input signals A, B,and C to the PVC sense amplifier inputs within sense amplifiers circuit360′. A fourth PVC sense vector B-(C-A) can be mathematically derivedfrom the input signals A, B and C, but the simpler selection of a pairof input signals among signals A, B, and C may well suffice in practiceand will be assumed in the following description.

[0098] The optimal far field sense vector for sensing an R-wave, and, bylogical extension, for sensing PVCs occurring during the PAVBP can bedetermined following implantation of the IPG 100′ and the leads 114 and116 in the patient's body. The sensitivity of the PVC sense amplifierand the V-SELECT pairing signals A, B, and C can both be temporarilyprogrammed in a telemetry session with an external programmer, and theIPG 100′ can be commanded to uplink telemeter the PVC sense signal. Theintrinsic R-waves and any spontaneously occurring PVCs appearing in aconventional ECG display and the PVC sense amplifier's uplinktelemetered response can be observed simultaneously. The PVC senseamplifier sensitivity can be varied for each programmed far field sensevector, and the sense vector providing the best consistent detection ofR-waves with the best ventricular sense safety margin can be determined.Other comparative tests to determine the optimal PVC sense vector couldinclude simply measuring the R wave amplitude, the R wave width, andslew rates through the PVC sense amplifier and determining the optimalPVC sense amplifier through comparison of one or a combination of theseparameters of the sensed R-waves. Or comparative testing can beconducted varying a blanking period applied to the PVC sense amplifierto determine the PVC sense vector across which an R-wave can be sensedat the longest blanking period. A “permanent” V-SELECT can then beprogrammed for coupling the PVC sense amplifier inputs to receive theoptimal pair of signals A, B, C during chronic implantation.

[0099] The chronic operation of the selected far field PVC sense vectorcan be determined in a telemetry session initiated at a later time fromdata accumulated in memory registers indicating the number of times thata PVC was detected during the PAVBP and the delivery of a V-PACE wasinhibited at time-out of the PAV delay. In IPGs having the VSP function,the saved data would comprise the number of times that a PVC wasdetected during the PAVBP and/or before time-out of the VSP delay, andthe delivery of a V-PACE pulse at time-out of the VSP delay.

[0100] In a similar way, optimal PVC sense vectors can be selected in adual chamber pacing systems providing right and left heart chamberpacing and sensing of the type described in commonly assigned U.S. Pat.No. 6,477,415. Such multi-chamber pacing systems provide right and leftatrial and/or ventricular pacing and sensing particularly to enhancecardiac output of hearts in heart failure. In such a right and leftheart pacing context providing atrial pacing, the PVC sense electrodepair can comprise right and left heart chamber pace/sense electrodesdefining an optimal R-wave sense vector or can additionally comprise oneof the left heart chamber pace/sense electrodes and the indifferentelectrode supported on or comprising the conductive IPG can defining anoptimal PVC sense vector.

[0101] Such a right and left heart pacing system comprising endocardialRV lead 114, RA lead 116, and a CS lead 150 transvenously introducedinto the right ventricle 16, the right atrium 12, and the coronarysinus, respectively of the heart 10 and coupled to the connector block112″ of the IPG 100′″ is depicted in FIGS. 7 and 8. The depicted CS lead150 supports an LV pace/sense electrode 154 disposed in the CS or acoronary vein descending from the CS in operative relation to the LV andan LA pace/sense electrode 152 disposed in the CS in operative relationto the LA.

[0102] Pacing and sensing in the RA and RV one or both of the LA and LVcan be conducted in the manner described in the above-referenced '415patent. The components of the IPG circuit 300′″ correspond in large partwith the components of the IPG circuit 300 described above. The flowcharts of FIGS. 3 and 4 are followed, and right and left heart pacingpulses can be delivered simultaneously or with a delay as determined inblock 364 of digital controller/timer circuit 330. The output amplifierscircuit 340′ can deliver the depicted RA-PACE, LA-PACE, RA-PACE andRV-PACE pulses through selected pace/sense electrode pairs or employingthe can electrode 140 as an indifferent pacing electrode.

[0103] Similarly, the sense amplifiers circuit 360′ includes therespective atrial and ventricular sense amplifiers for declaring anLA-EVENT, an RA-EVENT, an LV-EVENT or an RV-EVENT through selectedpace/sense electrode pairs or employing the can electrode 140 as anindifferent sense electrode.

[0104] A PVC sense vector can be defined by an appropriate V-SELECTcommand through pace/sense electrode selection and control circuit orregisters 350′. In this embodiment illustrated in FIGS. 7 and 8, the PVCsense vector can be selected by an appropriate V-SELECT command among:(1) the can electrode 140 and the RV ring pace/sense electrode 128; (2)the can electrode 140 and the LV pace/sense electrode 154; (3) the LApace/sense electrode 152 and the LV pace/sense electrode 154; (4) the LApace/sense electrode 152 and the RV ring pace/sense electrode 128; and(5) the RV ring pace/sense electrode 128 the LV pace/sense electrode 154depicted as PVC sense vector 160 in FIG. 7. The selection can be madeemploying comparative testing of the PVC sense electrode pairs asdescribed above.

[0105] In a similar way, PVC sense vectors can be selected in a dualchamber pacing ICD implanted in a patient's chest comprising an ICD IPGand endocardial leads transvenously introduced into the right atrium,right ventricle, and coronary sinus of the heart bearing pace/senseand/or cardioversion/defibrillation electrodes, wherein PVC sensing canbe conducted during the PVAB period across selected far field PVC senseelectrode pairs. The dual chamber ICD can also be configured to provideright and left heart pacing and/or have at lease one SEA electrodeprovided with the as in the embodiments of FIGS. 5-8.

[0106]FIGS. 9 and 10 illustrate a dual chamber, multi-programmable, ICDIPG 400 and associated lead system for providing atrial and/orventricular sensing functions for detecting P-waves of atrialdepolarizations and/or R-waves of ventricular depolarizations, dependingon the programmed pacing and/or sensing mode and delivering pacing orcardioversion/defibrillation therapies. An exemplarycardioversion/defibrillation lead system is depicted in FIG. 9 fordelivering cardioversion/defibrillation shock therapies to the atria orventricles of the heart. FIGS. 9 and 10 are intended to provide acomprehensive illustration of each of the atrial and/or ventricular,pacing and/or cardioversion/defibrillation configurations that may beeffected using sub-combinations of the components depicted therein andequivalents thereto. The present invention can be implemented into suchICDs wherein the R-wave sense amplifier is normally blanked during aPAVBP following delivery of an A-PACE pulse.

[0107] In the preferred embodiment of FIGS. 9 and 10, depending on theprogrammed or current pacing mode, pacing pulses are applied to theatrium and/or ventricle in response to the detection of the appropriatebradycardia condition by the ICD IPG operating system. The pacing andsensing functions are effected through atrial and ventricular bipolarpace/sense electrode pairs at the ends of right atrial/superior venacava (RA/SVC) and right ventricular (RV) leads 440 and 416,respectively, fixed in the right atrium 12 and right ventricle 16,respectively, that are electrically coupled to the circuitry of IPG 400through a connector block 412. Delivery of cardioversion ordefibrillation shocks to the atrial and/or ventricular chambers of theheart 10 may be effected through selected combinations of theillustrated exemplary RA and RV cardioversion/defibrillation electrodeson the RA/SVC and RV leads and an additional coronary sinus (CS)electrode on a CS lead 430 as well as an exposed surface electrode 410of the outer housing or can of the IPG 400. The can electrode 410optionally serves as a subcutaneous cardioversion/defibrillationelectrode, used as one electrode optionally in combination with oneintracardiac cardioversion/defibrillation electrode for cardioverting ordefibrillating either the atria or ventricles. A remote, subcutaneousdefibrillation patch electrode may be provided in addition to orsubstitution for the can electrode 410.

[0108] The RV lead 416 is depicted in a conventional configuration andincludes an elongated insulating lead body, enclosing three concentric,electrically isolated, coiled wire conductors, separated from oneanother by tubular insulating sheaths. Located adjacent the distal endof the RV lead 416 are a pace/sense ring electrode 424, a helical,pace/sense electrode 426, mounted retractably within an insulatingelectrode head 428. Helical electrode 426 is adapted to be extended outof the electrode head 428 and screwed into the ventricular apex in amanner well known in the art. RV pace/sense electrodes 424 and 426 areeach coupled to a coiled wire conductor within the RA lead body and areemployed for cardiac pacing in the ventricle and for sensing near-fieldR-waves. RV lead 416 also supports an elongated, exposed wire coil,cardioversion/defibrillation electrode 422 in a distal segment thereofadapted to be placed in the right ventricle 16 of heart 10. The RVcardioversion/defibrillation electrode 422 may be fabricated fromplatinum, platinum alloy or other materials known to be usable inimplantable cardioversion/defibrillation electrodes and may be about 5cm in length. cardioversion/defibrillation electrode 422 is also coupledto one of the coiled wire conductors within the lead body of RV lead416. At the proximal end of the lead body is a bifurcated connector end418 having three exposed electrical connectors, each coupled to one ofthe coiled conductors that are attached within the connector block 412to connector block terminals in a manner well known in the art.

[0109] The coronary sinus (CS) lead 430 includes an elongated insulatinglead body enclosing one elongated coiled wire conductor coupled to anelongated exposed coil wire cardioversion/defibrillation electrode 434.CS cardioversion/defibrillation electrode 434, illustrated in brokenoutline, is located within the coronary sinus and great vein 408 of theheart 10 and may be about 5 cm in length. At the proximal end of the CSlead 430 is a connector end 432 having an exposed connector coupled tothe coiled wire conductor and attached within the connector block 412 toconnector block terminals in a manner well known in the art.

[0110] The RA/SVC lead 440 includes an elongated insulating lead bodycarrying three concentric, electrically isolated, coiled wire conductorsseparated from one another by tubular insulating sheaths, correspondinggenerally to the structure of the RV lead 416. The lead body is formedin a manner well known in the art in an atrial J-shape in order toposition its distal end in the right atrial appendage. A pace/sense ringelectrode 444 and an extendable helical, pace/sense electrode 446,mounted retractably within an insulating electrode head 448, are formeddistally to the bend of the J-shape. Helical electrode 446 is adapted tobe extended out of the electrode head 448 and screwed into the atrialappendage in a manner well known in the art. RA pace/sense electrodes444 and 446 are employed for atrial pacing and for near-field sensing ofP-waves. An elongated, exposed coil defibrillation RA/SVC electrode 450is supported on RA lead 440 extending proximally to pace/sense ringelectrode 444 and coupled to the third coiled wire conductor within theRA lead body. Electrode 450 preferably is 40 cm in length or greater andis configured to extend from within the SVC and toward the tricuspidvalve. At the proximal end of the RA lead 440 is a bifurcated connector442 which carries three exposed electrical connectors, each coupled toone of the coiled wire conductors, that are attached within theconnector block 412 to connector block terminals in a manner well knownin the art.

[0111] Preferably, bipolar pace/sense electrodes 444, 446 and 424, 426are employed for near field sensing and for delivery of pacing pulses tothe atria and ventricles. The configuration, manner of fixation, andpositioning of bipolar pace/sense electrodes 444, 446 and 424, 426 withrespect to the atria and ventricles, respectively, may differ from thoseshown in FIG. 9. Unipolar pace/sense electrode bearing leads may also beused in the practice of the invention, and the second, return electrodemay be one or more of the cardioversion/defibrillation electrodes or thecan electrode 410.

[0112] The ICD system configuration and operating modes of FIG. 9 may bevaried by eliminating: (1) the atrial or ventricularcardioversion/defibrillation capability and associated lead andelectrodes while retaining the dual chamber pacing and sensingcapability thereby providing single chamber cardioversion/defibrillationand dual chamber bradycardia/tachycardia pacing capabilities; or (2) ina special case of an atrial ICD, the ventricularcardioversion/defibrillation capability while retaining at least theatrial pace/sense capability and the ventricular sense capability forproviding R-wave synchronization of the delivered atrial cardioversiontherapies. In each such system, it will be understood that appropriatedefibrillation and pacing leads will be employed in the system. In asimpler ICD system employing only the IPG can electrode 410 or acardioversion/defibrillation electrode implanted subcutaneously and moreremote from the heart chamber and only one the other of thecardioversion/defibrillation electrode located in proximity to theatrium or ventricle, e.g. electrodes 422 or 450, then it is desirable tocouple the PVC sense amplifier inputs to the availablecardioversion/defibrillation electrodes.

[0113]FIG. 10 is a functional schematic diagram of the circuitry of adual chamber, ICD 400 in which the present invention may usefully bepracticed. The circuitry of FIG. 10 should be taken as exemplary of adual chamber ICD IPG 400 in which the invention may be embodied, and notas limiting, as it is believed that the invention may usefully bepracticed in a wide variety of device implementations, as long as a dualchamber pacing mode providing bradycardia pacing therapies to the atriais retained that involve blanking of the ventricular sense amplifier.

[0114] The ICD IPG circuitry of FIG. 10 includes a high voltage sectionfor providing relatively high voltage cardioversion/defibrillationshocks when needed in response to detection of a tachyarrhythmia, a lowvoltage pace/sense section for sensing P-waves and/or R-waves andproviding relatively low voltage bradycardia pacing and anti-tachycardiapacing therapies, both operated under the control of a microcomputerincluding a microprocessor 224, ROM/RAM 226 and DMA 228. Otherfunctions, including uplink and downlink telemetry with an externalprogrammer for interrogating or programming operating modes andparameters, are also provided (but not shown) in a manner well known inthe art.

[0115] The block diagram of FIG. 10 depicts the atrial and ventricularpace/sense and defibrillation lead connector terminals of the connectorblock 412. Assuming the electrode configuration of FIG. 9, thecorrespondence to the illustrated leads and electrodes is as follows:Optional terminal 310 is hard wired to can electrode 410, that is, theun-insulated portion of the housing of the ICD IPG 400, and technicallymay be directly connected and not be part of the connector block 412.Terminal 320 is adapted to be coupled through RV lead 416 to RVcardioversion/cardioversion/defibrillation electrode 422. Terminal 311is adapted to be coupled through RA lead 440 to RA/SVC electrode 450.Terminal 318 is adapted to be coupled through CS lead 430 to CScardioversion/defibrillation electrode 434. However, it will beunderstood that fewer terminals may be provided than depicted, and/orthat one or more differing defibrillation leads, e.g. epicardial patchelectrode and subcutaneous patch electrode bearing leads may also beemployed for one or more of the depicted cardioversion/defibrillationelectrode bearing leads.

[0116] Terminals 310, 311, 318 and 320 are coupled to high voltageoutput circuit 234. High voltage output circuit 234 includes highvoltage switches controlled by CV/DEFIB CONTROL logic 230 via controlbus 238. The switches within circuit 234 control which electrodes areemployed and which are coupled to the positive and negative terminals ofthe capacitor bank including capacitors 246 and 248 during delivery ofthe intermediate and high voltage cardioversion and defibrillationshocks.

[0117] Terminals 324 and 326 of the connector block are adapted to becoupled through RV lead 416 to RV pace/sense electrodes 424 and 426 forsensing and pacing in the ventricle. Terminals 317 and 321 are adaptedto be coupled through RA/SVC lead 440 to RA pace/sense electrodes 444and 446 for sensing and pacing in the atrium. Terminals 324 and 326 arecoupled to the inputs of R-wave sense amplifier 200 through switches inswitch network 208. R-wave sense amplifier 200, which preferably takesthe form of an automatic gain controlled amplifier providing anadjustable sensing threshold as a function of the measured R-wave signalamplitude. A VSENSE signal is generated on R-OUT line 202 whenever thesignal sensed between electrodes 424 and 426 exceeds the currentventricular sensing threshold.

[0118] Terminals 317 and 321 are coupled to the P-wave sense amplifier204 through switches in switch network 208. P-wave sense amplifier 204preferably also takes the form of an automatic gain controlled amplifierproviding an adjustable sensing threshold as a function of the measuredP-wave amplitude. An ASENSE signal is generated on P-OUT line 206whenever the signal sensed between pace/sense electrodes coupled toterminals 317, 321 exceeds the current atrial sensing threshold.

[0119] The A-PACE and V-PACE output circuits 214 and 216 are alsocoupled to terminals 317, 321 and 324, 326, respectively. The atrial andventricular sense amplifiers 204 and 200 are isolated from the A-PACEand V-PACE output circuits 214 and 216 by appropriate isolation switcheswithin switch matrix 208 and also by blanking circuitry operated byA-BLANK and V-BLANK signals during and for a short time followingdelivery of a pacing pulse in a manner well known in the art. One of theV-BLANK signals is the post atrial ventricular blanking signal providedduring the PAVBP period as described above with reference to the dualchamber pacemaker IPG 100. The general operation of the R-wave andP-wave sense amplifiers 200 and 204 may correspond to that disclosed inU.S. Pat. No. 5,117,824, for example.

[0120] The ICD IPG circuitry of FIG. 10 provides atrial and/orventricular cardiac pacing for bradycardia and tachycardia conditionsand synchronized cardioversion and defibrillation shock therapies fortachyarrhythmias in accordance with therapy regimes programmed by thephysician. With respect to the pacing operations, the pacer timing andcontrol circuitry 212 includes programmable digital counters whichcontrol the basic time intervals associated with bradycardia pacingmodes as is well known to the art.

[0121] In normal pacing modes of operation, e.g., the dual chamberpacing mode as set forth in FIG. 3, for example, intervals defined bypacer timing and control circuitry 212 include atrial and ventricularpacing escape intervals, blanking intervals, including the PAVBP, therefractory periods during which sensed P-waves and R-waves areineffective to restart timing of the escape intervals, and the pulsewidths of the pacing pulses. These intervals are determined bymicroprocessor 224, in response to stored data in RAM in ROM/RAM 226 andare communicated to the pacer timing and control circuitry 212 viaaddress/data bus 218. Pacer timing and control circuitry 212 alsodetermines the amplitude of the cardiac pacing pulses under control ofmicroprocessor 224.

[0122] During pacing, the escape interval counters within pacer timingand control circuitry 212 are reset upon sensing of R-waves and P-wavesas indicated by a signals on lines 202 and 206. In accordance with theselected pacing mode, pacer timing and control circuitry 212 providespace trigger signals to the A-PACE and V-PACE output circuits 214 and216 on timeout of the appropriate escape interval counters to triggergeneration of atrial and/or ventricular pacing pulses. The pacing escapeinterval counters are also reset on generation of A-PACE and R-PACEpulses, and thereby control the basic timing of cardiac pacingfunctions.

[0123] Pacer timing and control circuitry 212 also controls escapeintervals associated with timing and delivering anti-tachyarrhythmiapacing in both the atrium and the ventricle, employing anyanti-tachyarrhythmia pacing therapies known to the art. The value of thecounts present in the escape interval counters when reset by sensedR-waves and P-waves may be used as measures of the durations of R-Rintervals, P-P intervals, P-R intervals and R-P intervals, whichmeasurements are stored in RAM in ROM/RAM 226 and used to detect thepresence of tachyarrhythmias as described below.

[0124] Microprocessor 224 operates as an interrupt driven device, and isresponsive to interrupts from pacer timing and control circuitry 212corresponding to the occurrence sensed P-waves (ASENSE) and R-waves(VSENSE) and corresponding to the generation of cardiac pacing pulses.These interrupts are provided via data/address bus 218. Any necessarymathematical calculations to be performed by microprocessor 224 and anyupdating of the values or intervals controlled by pacer timing/controlcircuitry 212 take place following such interrupts.

[0125] For example, in response to a sensed or paced ventriculardepolarization or R-wave, the intervals separating that R-wave from theimmediately preceding R-wave, paced or sensed (R-R interval) and theinterval separating the paced or sensed R-wave from the preceding atrialdepolarization, paced or sensed (P-R interval) may be stored. Similarly,in response to the occurrence of a sensed or paced atrial depolarization(P-wave), the intervals separating the sensed P-wave from theimmediately preceding paced of sensed atrial contraction (P-P Interval)and the interval separating the sensed P-wave from the immediatelypreceding sensed or paced ventricular depolarization (R-P interval) maybe stored. Preferably, a portion of RAM in the ROM/RAM 226 (FIG. 10) isconfigured as a plurality of recirculating buffers, capable of holding apreceding series of measured intervals, which may be analyzed inresponse to the occurrence of a pace or sense interrupt to determinewhether the patient's heart is presently exhibiting atrial orventricular tachyarrhythmia.

[0126] Detection of atrial or ventricular tachyarrhythmias, as employedin the present invention, may correspond to tachyarrhythmia detectionalgorithms known to the art. For example, presence of atrial orventricular tachyarrhythmia may be confirmed by means of detection of asustained series of short R-R or P-P intervals of an average rateindicative of tachyarrhythmia or an unbroken series of short R-R or P-Pintervals. The suddenness of onset of the detected high rates, thestability of the high rates, or a number of other factors known to theart may also be measured at this time.

[0127] In the event that an atrial or ventricular tachyarrhythmia isdetected, and an anti-tachyarrhythmia pacing regimen is prescribed,appropriate timing intervals for controlling generation ofanti-tachyarrhythmia pacing therapies are loaded from microprocessor 224into the pacer timing and control circuitry 212, to control theoperation of the escape interval counters therein and to definerefractory periods during which detection of R-waves and P-waves isineffective to restart the escape interval counters.

[0128] In the event that generation of a cardioversion or defibrillationshock is required, microprocessor 224 employs the an escape intervalcounter to control timing of such cardioversion and defibrillationpulses, as well as associated refractory periods. In response to thedetection of atrial or ventricular fibrillation or tachyarrhythmiarequiring a cardioversion pulse, microprocessor 224 activatescardioversion/defibrillation control circuitry 230, which initiatescharging of the high voltage capacitors 246 and 248 via charging circuit236, under control of high voltage charging control line 240. Thevoltage on the high voltage capacitors is monitored via VCAP line 244,and the monitored voltage signal is passed through multiplexer 220,digitized, and compared to a predetermined value set by microprocessor224 in ADC/comparator 222. When the voltage comparison is satisfied, alogic signal on Cap Full (CF) line 254 is applied tocardioversion/defibrillation control circuit 230, terminating charging.Thereafter, timing of the delivery of the defibrillation orcardioversion shock is controlled by pacer timing/control circuitry 212.Following delivery of the fibrillation or tachycardia therapy, themicroprocessor 224 then returns the operating mode to cardiac pacing andawaits the next successive interrupt due to pacing or the occurrence ofa sensed atrial or ventricular depolarization.

[0129] In the illustrated ICD operating system, delivery of thecardioversion or defibrillation shocks is accomplished by output circuit234, under control of control circuitry 230 via control bus 238. Outputcircuit 234 determines whether a monophasic or biphasic shock isdelivered, the polarity of the electrodes and which electrodes areinvolved in delivery of the shock. Output circuit 234 also includes highvoltage switches that control whether electrodes are coupled togetherduring delivery of the shock. Alternatively, electrodes intended to becoupled together during the shock may simply be permanently coupled toone another, either exterior to or interior of the device housing, andpolarity may similarly be pre-set, as in current implantabledefibrillators. An example of output circuitry for delivery of biphasicshock regimens to multiple electrode systems may be found in U.S. Pat.No. 4,727,877, for example.

[0130] In accordance with the present invention, a PVC sense amplifier210 is incorporated into the circuitry of FIG. 10 having a pair of senseinputs that can be selectively coupled through switches within switchnetwork 208 in response to a programmed V-SELECT command receivedthrough bus 218 to a pair of PVC sense electrodes selected from amongthe depicted electrodes, preferably from amongcardioversion/defibrillation electrodes 422, 434, 450, can electrode410, and one of the ring pace-sense electrode 424 and the tip pace/senseelectrode 426. Switch matrix 208 is used in the PVC sensing function ofthe present invention to select which pair of the available pace/senseand/or cardioversion/defibrillation electrodes is coupled to the inputsof wide band (0.5-200 Hz) PVC sense amplifier 210 for use in detectingPVCs during the PAVBP (and at other times during the cardiac cycle). APVC signal from bandpass amplifier 210 is passed through multiplexer 220may be converted to multi-bit digital signals by A/D converter 222, forstorage in RAM in ROM/RAM 226 under control of DMA 228. Themicroprocessor 224 may employ digital signal and morphology analysistechniques to characterize the digitized signals stored in ROM/RAM 226to recognize and classify the patient's heart rhythm employing any ofthe numerous signal processing methodologies known to the art.

[0131] In a dual chamber pacing mode involving atrial pacing andventricular sensing, the PVC amplifier 210 is not blanked during thePAVBP. The steps set forth in FIGS. 3 and 4 are followed. A PVC that isdetected during time-out of a PAV is employed as an interrupt to themicroprocessor 224 in step S130.

[0132] The steps of FIG. 4 are followed to determine whether to inhibitthe delivery of a V-PACE pulse upon time-out of the PAV delay or todeliver a V-PACE upon time out of a VSP delay.

[0133] In this embodiment illustrated in FIGS. 9 and 10, the PVC sensevector can be selected by an appropriate V-SELECT command among: (1) thecan electrode 410 and the RV coil cardioversion/defibrillation electrode422; (2) the can electrode 410 and the SVC coilcardioversion/defibrillation electrode 450; (3) the can electrode 410and the RV ring pace/sense electrode 424; (4) the can electrode 410 andthe CS coil cardioversion/defibrillation electrode 434; (5) the CS coilcardioversion/defibrillation electrode 434 and the RV ring pace/senseelectrode 424; (6) the CS coil cardioversion/defibrillation electrode434 and the RV coil cardioversion/defibrillation electrode 422; and (7)the RV coil cardioversion/defibrillation electrode 422 and the SVC coilcardioversion/defibrillation electrode 450.

[0134] Advantageously, the PVC sense amplifiers within the senseamplifiers circuits 360, 360′, and 360″ and the PVC sense amplifier 210and can be enabled during the cardiac cycle to function as aconventional EGM sense amplifier so that the spontaneously occurringPQRST complexes can be recorded for real time analysis or data storageas is well known in the art.

[0135] All patents and publications referenced herein are herebyincorporated by reference in their entireties.

[0136] It will be understood that certain of the above-describedstructures, functions and operations of the above-described preferredembodiments are not necessary to practice the present invention and areincluded in the description simply for completeness of an exemplaryembodiment or embodiments.

[0137] In addition, it will be understood that specifically describedstructures, functions and operations set forth in the above-referencedpatents can be practiced in conjunction with the present invention, butthey are not essential to its practice. It is therefore to beunderstood, that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described withoutactually departing from the spirit and scope of the present invention.

What is claimed is:
 1. In a pacing system comprising an implantablepulse generator adapted to be implanted in a patient's body, an atriallead extending from the implantable pulse generator having at least oneactive atrial pace/sense electrode adapted to be disposed in operativerelation to an atrial heart chamber, at least one indifferent atrialpace/sense electrode adapted to be implanted in the patient's body, aventricular lead extending from the implantable pulse generator havingat least one active ventricular pace/sense electrode adapted to bedisposed in operative relation to a ventricular heart chamber, and anindifferent ventricular pace/sense electrode adapted to be implanted inthe patient's body, the pacing system further comprising: atrial pacepulse generator means coupled to the active and indifferent atrial pacesense electrodes for generating and delivering atrial pace (A-PACE)pulses to the atrial heart chamber; first ventricular sensing meanscoupled to the active and indifferent ventricular pace/sense electrodesfor sensing natural ventricular depolarizations and declaring a V-EVENT;ventricular pace pulse generator means coupled to the active andindifferent ventricular pace/sense electrodes for generating anddelivering ventricular pace (V-PACE) pulses to the ventricular heartchamber; V-A interval timing means for timing out a V-A intervalfollowing generation of a V-PACE pulse by said ventricular pulsegenerator means and following a V-EVENT declared by the ventricularsensing means; means for triggering said atrial pulse generator means togenerate an A-PACE pulse at the expiration of the V-A interval; meansfor timing out a PAV interval upon triggering said atrial pulsegenerator means to generate an A-PACE pulse at the expiration of the V-Ainterval; and ventricular blanking means for preventing said firstventricular sense amplifier from declaring a V-EVENT for a predeterminedventricular blanking period following generation and delivery of anatrial pace pulse; the improvement for declaring and responding to apremature ventricular contraction (PVC) comprising: a PVC senseelectrode pair disposed in the patient's body defining a PVC sensevector differing from a ventricular sense vector defined by the activeand indifferent ventricular pace/sense electrodes; PVC sensing meanscoupled to the PVC sense electrode pair for sensing a depolarization ofthe ventricles occurring during the ventricular blanking period anddeclaring a PVC; and means responsive to the declared PVC forterminating the PAV delay and triggering the V-A interval timing meansto time out the V-A interval.
 2. The pacing system of claim 1, furthercomprising: means for timing a ventricular safety pace delay from thedelivery of an A-PACE pulse; and means responsive to a declaration of aPVC for triggering the ventricular pace pulse generating means togenerate and deliver a V-PACE pulse through the active and indifferentventricular pace/sense electrodes to the ventricular heart chamber upontime-out of the ventricular safety pace delay.
 3. The pacing system ofclaim 2, wherein the ventricular safety pace delay is longer than theventricular blanking period and is shorter than the PAV delay and isselected to ensure that the delivered V-PACE pulse is not delivered intothe vulnerable period of the heart.
 4. The pacing system of claim 1,further comprising: atrial sensing means coupled to the active andindifferent atrial pace/sense electrodes for sensing natural atrialdepolarizations and declaring an A-EVENT; means for terminating the V-Ainterval upon declaration of an A-EVENT during time-out of the V-Ainterval; and means for timing out an SAV interval upon declaration ofan A-EVENT during time-out of the V-A interval.
 5. The pacing system ofclaim 1, further comprising: means for timing a ventricular safety pacedelay from the delivery of an A-PACE pulse, the ventricular safety pacedelay is longer than the ventricular blanking period and is shorter thanthe PAV delay and is selected to ensure that the delivered V-PACE pulseis not delivered into the vulnerable period of the heart; and meansresponsive to a declaration of a PVC during the ventricular safety pacedelay for triggering the ventricular pace pulse generating means togenerate and deliver a V-PACE pulse through the active and indifferentventricular pace/sense electrodes to the ventricular heart chamber upontime-out of the ventricular safety pace delay.
 6. The pacing system ofclaim 1, wherein: the active and indifferent ventricular pace/senseelectrodes are disposed on the ventricular lead; and the PVC senseelectrode pair comprises an indifferent pace/sense electrode disposed onthe implantable pulse generator and one of the active and indifferentventricular pace/sense electrodes are disposed on the ventricular lead.7. The pacing system of claim 1, wherein: the implantable pulsegenerator comprises a housing supporting at least two sense electrodesin a sense electrode array; and the PVC sense electrode pair comprisesthe sense electrodes supported by the implantable pulse generatorhousing.
 8. The pacing system of claim 1, wherein the implantable pulsegenerator comprises a housing supporting at least three sense electrodesin a sense electrode array, and further comprising: means of selectingthe PVC sense electrode pair from among the at least three senseelectrodes supported by the implantable pulse generator housing.
 9. Thepacing system of claim 1, further comprising a first and secondcardioversion/defibrillation electrodes disposed about one of the atriaand the ventricles for delivering cardioversion/defibrillation shocks tothe heart chamber, and wherein: the implantable pulse generator furthercomprises means for determining the existence of a tachyarrhythmia andproviding cardioversion/defibrillation shock therapy through the firstand second cardioversion/defibrillation electrodes; and the PVC senseelectrodes include at least one of the first and secondcardioversion/defibrillation electrodes.
 10. The pacing system of claim9, wherein the PVC sense electrodes include the first and secondcardioversion/defibrillation electrodes.
 11. The pacing system of claim1, wherein: the atrial lead supports a firstcardioversion/defibrillation electrode adapted to be disposed inrelation to the heart, and the ventricular lead supports a secondcardioversion/defibrillation electrode adapted to be disposed inrelation to the heart; the implantable pulse generator further comprisesmeans for determining the existence of a tachyarrhythmia and providingcardioversion/defibrillation shock therapy through the first and secondcardioversion/defibrillation electrodes; and the PVC sense electrodesinclude at least one of the first and secondcardioversion/defibrillation electrodes.
 12. The pacing system of claim11, wherein the PVC sense electrodes include the first and secondcardioversion/defibrillation electrodes.
 13. The pacing system of claim1, further comprising a coronary sinus lead extending from theimplantable pulse generator supporting a firstcardioversion/defibrillation electrode adapted to be disposed inrelation to a left heart chamber of the heart, and wherein: theventricular lead supports a second cardioversion/defibrillationelectrode adapted to be disposed in relation to the right ventricle ofthe heart; the implantable pulse generator further comprises means fordetermining the existence of a tachyarrhythmia and providingcardioversion/defibrillation shock therapy through the first and secondcardioversion/defibrillation electrodes; and the PVC sense electrodesinclude at least one of the first and secondcardioversion/defibrillation electrodes.
 14. The pacing system of claim13, wherein the PVC sense electrodes include the first and secondcardioversion/defibrillation electrodes.
 15. The pacing system of claim1, further comprising a coronary sinus lead extending from theimplantable pulse generator supporting a left heart chamber pace/senseelectrode adapted to be disposed in relation to a left heart chamber ofthe heart, and wherein: the implantable pulse generator furthercomprises means for providing synchronized pacing of right and leftheart chambers through the atrial and ventricular pace/sense electrodesand the left heart chamber pace/sense electrode; and the PVC senseelectrodes include the left heart chamber pace/sense electrode.
 16. Thepacing system of claim 15, wherein the PVC sense electrodes include theleft heart chamber pace/sense electrode and one of the active andindifferent ventricular pace/sense electrodes.
 17. The pacing system ofclaim 15, wherein the PVC sense electrodes include the left heartchamber pace/sense electrode an indifferent pace/sense electrodedisposed on the implantable pulse generator.
 18. In a pacing systemcomprising an implantable pulse generator adapted to be -implanted in apatient's body, an atrial lead extending from the implantable pulsegenerator having at least one active atrial pace/sense electrode adaptedto be disposed in operative relation to an atrial heart chamber, atleast one indifferent atrial pace/sense electrode adapted to beimplanted in the patient's body, a ventricular lead extending from theimplantable pulse generator having at least one active ventricularpace/sense electrode adapted to be disposed in operative relation to aventricular heart chamber, and an indifferent ventricular pace/senseelectrode adapted to be implanted in the patient's body, the pacingsystem further comprising: atrial sensing means coupled to the activeand indifferent atrial pace/sense electrodes for sensing natural atrialdepolarizations and declaring an A-EVENT; atrial pace pulse generatormeans coupled to the active and indifferent atrial pace sense electrodesfor generating and delivering atrial pace (A-PACE) pulses to the atrialheart chamber; ventricular sensing means coupled to the active andindifferent ventricular pace/sense electrodes for sensing naturalventricular depolarizations and declaring a V-EVENT; ventricular pacepulse generator means coupled to the active and indifferent ventricularpace/sense electrodes for generating and delivering ventricular pace(V-PACE) pulses to the ventricular heart chamber; V-A interval timingmeans for timing out a V-A interval following generation of a V-PACEpulse by said ventricular pulse generator means and following a V-EVENTdeclared by the ventricular sensing means; means for triggering saidatrial pulse generator means to generate an A-PACE pulse at theexpiration of the V-A interval; means for timing out a PAV interval upontriggering said atrial pulse generator means to generate an A-PACE pulseat the expiration of the V-A interval; and ventricular blanking meansfor preventing said first ventricular sense amplifier from declaring aV-EVENT for a predetermined ventricular blanking period followinggeneration and delivery of an atrial pace pulse; a method of declaringand responding to a premature ventricular contraction (PVC) comprising:sensing a depolarization of the ventricles occurring during theventricular blanking period across a PVC sense electrode pair in thepatient's body defining a PVC sense vector differing from a ventricularsense vector defined by the active and indifferent ventricularpace/sense electrodes and declaring a PVC; and terminating the PAV delayand triggering the V-A interval timing means to time out the V-Ainterval means in response to the declared PVC.
 19. The method of claim18, further comprising: timing a ventricular safety pace delay from thedelivery of an A-PACE pulse; and in response to a declaration of a PVC,triggering the ventricular pace pulse generating means to generate anddeliver a V-PACE pulse through the active and indifferent ventricularpace/sense electrodes to the ventricular heart chamber upon time-out ofthe ventricular safety pace delay.
 20. The method of claim 19, whereinthe ventricular safety pace delay is longer than the ventricularblanking period and is shorter than the PAV delay and is selected toensure that the delivered V-PACE pulse is not delivered into thevulnerable period of the heart.
 21. The method of claim 18, furthercomprising: timing a ventricular safety pace delay from the delivery ofan A-PACE pulse, the ventricular safety pace delay is longer than theventricular blanking period and is shorter than the PAV delay and isselected to ensure that the delivered V-PACE pulse is not delivered intothe vulnerable period of the heart; and in response to a declaration ofa PVC during the ventricular safety pace delay, triggering theventricular pace pulse generating means to generate and deliver a V-PACEpulse through the active and indifferent ventricular pace/senseelectrodes to the ventricular heart chamber upon time-out of theventricular safety pace delay.
 22. The method of claim 18, wherein thepacing system further comprises atrial sensing means coupled to theactive and indifferent atrial pace/sense electrodes for sensing naturalatrial depolarizations and declaring an A-EVENT, and the method furthercomprises: terminating the V-A interval upon declaration of an A-EVENTduring time-out of the V-A interval; and timing out an SAV interval upondeclaration of an A-EVENT during time-out of the V-A interval.
 23. Themethod of claim 18, wherein: the active and indifferent ventricularpace/sense electrodes are disposed on the ventricular lead; and the PVCsense electrode pair comprises an indifferent pace/sense electrodedisposed on the implantable pulse generator and one of the active andindifferent ventricular pace/sense electrodes are disposed on theventricular lead.
 24. The method of claim 18, wherein: the implantablepulse generator comprises a housing supporting at least two senseelectrodes in a sense electrode array; and the PVC sense electrode paircomprises the sense electrodes supported by the implantable pulsegenerator housing.
 25. The method of claim 18, wherein the implantablepulse generator comprises a housing supporting at least three senseelectrodes in a sense electrode array, and further comprising: selectingthe PVC sense electrode pair from among the at least three senseelectrodes supported by the implantable pulse generator housing.
 26. Themethod of claim 18, further comprising a first and secondcardioversion/defibrillation electrodes disposed about one of the atriaand the ventricles for delivering cardioversion/defibrillation shocks tothe heart chamber, and wherein: the implantable pulse generator furthercomprises means for determining the existence of a tachyarrhythmia andproviding cardioversion/defibrillation shock therapy through the firstand second cardioversion/defibrillation electrodes; and the sensing stepcomprises sensing a depolarization of the ventricles occurring duringthe ventricular blanking period across the PVC sense electrode pairincluding at least one of the first and secondcardioversion/defibrillation electrodes.
 27. The method of claim 26,wherein the PVC sense electrodes include the first and secondcardioversion/defibrillation electrodes.
 28. The method of claim 18,wherein: the atrial lead supports a first cardioversion/defibrillationelectrode adapted to be disposed in relation to the heart, theventricular lead supports a second cardioversion/defibrillationelectrode adapted to be disposed in relation to the heart; theimplantable pulse generator further comprises means for determining theexistence of a tachyarrhythmia and providingcardioversion/defibrillation shock therapy through the first and secondcardioversion/defibrillation electrodes; and the sensing step comprisessensing a depolarization of the ventricles occurring during theventricular blanking period across the PVC sense electrode pairincluding at least one of the first and secondcardioversion/defibrillation electrodes.
 29. The method of claim 28,wherein the PVC sense electrodes include the first and secondcardioversion/defibrillation electrodes.
 30. The method of claim 18,further comprising a coronary sinus lead extending from the implantablepulse generator supporting a first cardioversion/defibrillationelectrode adapted to be disposed in relation to the heart, and wherein:the ventricular lead supports a second cardioversion/defibrillationelectrode adapted to be disposed in relation to the heart; theimplantable pulse generator further comprises means for determining theexistence of a tachyarrhythmia and providingcardioversion/defibrillation shock therapy through the first and secondcardioversion/defibrillation electrodes; and the sensing step comprisessensing a depolarization of the ventricles occurring during theventricular blanking period across the PVC sense electrode pairincluding at least one of the first and secondcardioversion/defibrillation electrodes.
 31. The method of claim 30,wherein the PVC sense electrodes include the first and secondcardioversion/defibrillation electrodes.
 32. The method of claim 18,further comprising: disposing a left heart chamber pace/sense electrodein relation to a left heart chamber of the heart, and providingsynchronized pacing of right and left heart chambers through the atrialand ventricular pace/sense electrodes and the left heart chamberpace/sense electrode; and wherein: the sensing step comprises sensing adepolarization of the ventricles occurring during the ventricularblanking period across the PVC sense electrode pair including the leftheart chamber pace/sense electrode.
 33. The method of claim 32, whereinthe sensing step comprises sensing a depolarization of the ventriclesoccurring during the ventricular blanking period across the PVC senseelectrode pair including the left heart chamber pace/sense electrode andone of the active and indifferent ventricular pace/sense electrodes. 34.The method of claim 32, wherein the sensing step comprises sensing adepolarization of the ventricles occurring during the ventricularblanking period across the PVC sense electrode pair including the leftheart chamber pace/sense electrode an indifferent pace/sense electrodedisposed on the implantable pulse generator.